A lithographic printing plate generally comprises a hydrophobic imaging area, which receives oily ink in printing, and a hydrophilic non-imaging area (ink-repelling area), which receives dampening water. The lithographic printing process is essentially based on the fact that water is incompatible with oily ink. In the process, the ink is put on a surface of the plate unevenly so that only the imaging area may receive it, and then transferred onto an object (e.g., paper) to be printed.
For producing a lithographic printing plate, a presensitized printing plate (PS plate) comprising a hydrophilic support and a thereon-provided hydrophobic photosensitive resin layer (image-recording layer) has been conventionally used. The PS plate is masked and exposed to light through an original image (e.g., a lith film), and then the image-recording layer in the non-imaging area is dissolved and removed with an alkaline developing solution or organic solvent to bare imagewise the hydrophilic surface of the support and to leave the image-recording layer in the imaging area.
In the conventional process for producing the printing plate, the step of dissolving and removing the image-recording layer in the non-imaging area is indispensable. However, in these days, it has been wanted to omit or simplify an auxiliary wet-process such as developing process. From the viewpoint of protecting global environment, the whole industrial world pays attention to disposal of wastewater exhausted in the wet-process. Accordingly, in consideration of environment, it has been more and more wanted to omit the wet-process.
As a simple process for making a printing plate, a method called “press development” is proposed. In the press development, a presensitized plate after exposed to light is installed in a printing machine and then the image-recording layer in the non-imaging area is removed while the plate stays on the machine to print. The presensitized plate used in this development must have an image-recording layer partly removal in the non-imaging area during a normal printing process.
In the press development, for example, the image-recording layer in the non-imaging area can be dissolved or dispersed in dampening water, solvent of ink, or an emulsion of dampening water and ink. The layer may be mechanically removed when brought into contact with rollers or a blanket cylinder of the printing machine. Before removed mechanically with the rollers or cylinder, it may be beforehand swollen with dampening water or solvent of ink to weaken the cohesion of the layer or the adhesion onto the support.
In the present specification, the term “developing process” means a procedure in which a presensitized plate having been exposed to an IR laser beam is brought into contact with liquid (normally, alkaline developing solution) in an apparatus (normally, automatic developing machine) other than a printing machine so that the image-recording layer in the non-imaging area, where the laser beam was not applied, may be removed to bare a surface of the hydrophilic support and, as a result, to form an image in relief. On the other hand, the term “press development”means another procedure in which a presensitized plate having been exposed to an IR laser beam is brought into contact with liquid (normally, printing ink and/or dampening water) on a printing machine so that the layer in the non-imaging area may be removed to bare a surface of the hydrophilic support and, as a result, to form an image in relief.
In a conventional image-recording process, ultraviolet or visible light has been normally used. Even after exposed to the light, an image-recording layer of presensitized plate used in the conventional process is not fixed. Therefore, if a conventional presensitized plate is used in the press development, it is necessary to adopt a complicated process comprising, for example, a step of keeping the exposed presensitized plate under completely light-shielded condition or at a constant temperature until the plate is installed in a printing machine.
Meanwhile, digital information technology has been spread recently. In a process based on the digital information technology, image information in the form of digital data is electronically processed, stored and outputted by means of computers. Accordingly, various new image-forming methods for the digitized process have been proposed and practically adopted. For example, a presensitized lithographic printing plate is exposed not through a lith film but directly to highly directive and active radiation, such as a laser beam, scanning according to the digital image information, to produce a printing plate. This plate-making method, which is called “Computer-To-Plate (CTP)”, has attracted the attention of people in this industrial field. Naturally, it is desired to provide a presensitized lithographic printing plate suitable for CTP.
As described above, in order to protect the global environment and to adapt to the digitization, it is desired more strongly than ever that the plate-making process be simplified and that the wet-process be omitted or converted to a dry-process.
Having recently been getting available at small cost, high-power lasers such as semi-conductor laser and YAG laser are expected to serve as image-recording means in a digitized plate-making process, in which a lithographic printing plate is produced by scanning exposure.
In the conventional process, the image-recoding layer is imagewise exposed to light of low or middle illuminance so that the properties of the layer material may be imagewise changed by photochemical reaction to record an image. On the other hand, in the process performed with a high-power laser, a great deal of photo-energy is applied to the area to be exposed for a very short period so that the photo-energy may be efficiently converted to thermal energy by which the properties (e.g., chemical characteristics, phase, form and/or structure) of the image-recoding layer in the exposed area are thermally changed to record an image. Accordingly, in the high-power laser process, image information is recorded by not only the photo-energy but also the thermal energy although inputted by only the photo-energy of laser beam. The recording process based on heat caused by exposure of high power density is generally called “heat-mode recording”, and the procedure of converting the photo-energy to the thermal energy is called “light-heat conversion”.
The plate-making process comprising the heat-mode recording has great advantages. For example, the image-recording layer used in the process is not sensitive to light of normal illuminance like room light, and it is not necessary to fix an image recorded with light of high illuminance in the image-recording layer. This means that there is no fear that room light impairs a presensitized lithographic printing plate used in the heat-mode recording and also that it is dispensable to fix the image recorded in the exposed presensitized plate. Accordingly, if an image-recording layer imagewise changed with a high-power laser to be soluble or insoluble is used in the plate-making process in which a lithographic printing plate is produced by the press development, the recorded image is not impaired even if the exposed image-recording layer is irradiated with room light. The heat-mode recording, therefore, is expected to make it possible to provide a presensitized lithographic printing plate suitable for the press development.
Laser technology has recently been remarkably developed, and high-power and downsized semi-conductor or solid-state lasers, which emit infrared rays in the wavelength range of 760 to 1,200 nm, have been getting easily available. These IR lasers are very suitably used as light-sources for recording images from which printing plates are directly produced according to digital date stored, for example, in computers.
However, since most practical photosensitive recording materials are sensitive to visible light of 760 nm or shorter, images cannot be recorded with IR lasers. It is, therefore, desired to provide a material so sensitive to infrared light that images can be recorded with IR lasers.
Japanese Patent No. 2,938,397 proposes a presensitized lithographic printing plate comprising a hydrophilic support and a thereon-provided image-forming layer made of hydrophilic binder and therein-dispersed particles of hydrophobic thermoplastic polymer to solve the above-mentioned problem. The proposed presensitized plate is exposed to an IR laser beam, to produce heat by which the particles of hydrophobic thermoplastic polymer are aggregated to form an image. The plate is then set on a cylinder of printing machine, and subjected to press development with printing ink and/or dampening water supplied.
The above process, in which the fine particles are simply aggregated by heat to form an image, is very suitable for the press development, but the formed image in relief (namely, the image-forming layer remaining in the image-forming area) has such poor durability (poor adhesion onto the support) that the plate wear is insufficient.
Japanese Patent Provisional Publication Nos. 2001-277740, 2001-277742 and 2002-137562 propose a presensitized lithographic printing plate comprising a hydrophilic support and a thereon-provided layer containing a polymerizable compound encased in microcapsules.
Japanese Patent Provisional Publication No. 2002-287334 proposes a presensitized lithographic printing plate comprising a support and a thereon-provided photosensitive layer containing an infrared absorber, a radical-polymerization initiator and a polymerizable compound.
The images in relief (imagewise remaining layers) given by the above processes, which are based on polymerization reactions, are made of substances having enough thickly formed chemical bonds to improve the durability, and accordingly they are stronger than the image in relief formed by melting and aggregating the fine particles of polymer. However, even so, from the practical viewpoint, they are still too unsatisfactory for the press development and have too poor plate wear and too low polymerization efficiency (sensitivity) to use practically.
Japanese Patent Provisional Publication No. 11(1999)-30858 proposes that a phosphoric ester having (meth)acryloyl group be incorporated as an additive into a presensitized lithographic printing plate. Japanese Patent Provisional Publication No. 10(1998)-333321 proposes another phosphoric ester as an additive for a presensitized lithographic printing plate to be subjected to the press development.